Snap acting switch



March 1, 1960 w. E. RHODES 2,927,171

SNAP ACTING SWITCH Filed Nov. 29, 1957 2 Sheets-Sheet 1 0 0 w w R m nIll-l 5 mm m M A m i .4233 W (P Q W/ y a 2 Q 1 I w W .1 v n a u u n h im/ 4 a a L'tllnvk l I I w 4:4 2/4; r E RH E mm v z ||lV1.l.\l1||| H M IHH. Y V/ 1 w w L a 5% w W w ATTORNEY March 1, 1960 w. E. RHODES2,927,171

SNAP ACTING SWITCH Filed Nov. 29, 1957 2 Sheets-Sheet 2 INVENTORATTORNEY 6-.0 44 .08 J? ./a .20 24 .26 .32 I a SNAP ACTING SWITCHWilliam E. Rhodes, Columbus, Ohio, assignor to Robertshaw-FultonControls Company, Richmond, Va., a corporation of Delaware ApplicationNovember 29, 1957, Serial No. 699,719

8 Claims. (Cl. 200-67) The present invention relates to a snap actingdevice, and more particularly to a new improved snap acting electricalswitch incorporating a novel improved type of arcuate operating springof special configuration.

United States Letters Patent to E. F. Kohl Nos. 2,237,705, 2,558,219 and2,558,258 disclose snap acting switches incorporating an arcuateoperating spring of rectangular outline. The aforesaid Kohl patentsdisclose that the arcuate operating spring for such switches can be ofnormally flat thin rectangular resilient metal strip bent into arcuateform when assembled in the switch, as well as preformed. However, it wasfound in practice that such a normally fiat rectangular spring wasusually not as good as the alternative preformed arcuate spring. Thereason for this is that when such an arcuate operating spring is bentfrom normally flat rectangular strip material upon assembly in theswitch, the end loading on the spring generally causes a sharpercurvature at the center of the spring are than at its sides. Thissharper curvature at the center of the arcuate spring provides alocalized high stress concentration similar to a notch effect, whichsometimes causes the spring to fracture in use, and generally shortensthe operating life of the spring and switch in any event. Moreover,although snap switches with such normally flat rectangular springs arefully satisfactory for some applications, the switch operatingcharacteristics such as contact pressure and operating force are not asprecise or controllable for high tolerance applications, and deterioraterelatively rapidly even if initially satisfactory, compared to a likeswitch having a carefully made preformed arcuate spring.

Accordingly, in making switches like those disclosed in theaforementioned Kohl patents, it has been the usual commercial practiceto preform the arcuate springs in such switches rather than use anormally flat spring. In making these arcuate springs in largequantities, the arcuate shape is obtained by bending the spring striparound a roller to give it a permanent deformation of desired arcuatecontour, or by an appropriate stamping operation. However, in massproduction of such preformed arcuate springs, it is difiicult toeliminate slight variations in the length of chord between the mountingends of the spring. Variations in chord length are also introduced byheat treatment of springs heretofore thus preformed, as amplified below.All other conditions being equal, these variations in chord length causea slight variation from switch to switch in such characteristics asoperating force, click point (i.e. operating snapover point), releaseforce, and contact pressure. Furthermore, preforming of the arcuatesprings and production control and inspection thereof are relativelycostly compared to the over-all cost of making Kohl type switches. Forexample, for close-tolerance precision switch applications, eachpreformed arcuate spring is tested with a spring tester to measure itsforce thus increasing switch production cost significantly.

In mass production, there is frequently a slight varianited StatesPatent tion from switch to switch in the spacing between the springsupporting shoulders of the actuating blade and the actuated contactblade, due to tolerances in these blade members and the switch stopswhich -control their operating positions. For some criticalclose-tolerance switch applications, these variations may introducesignificant changes in operating characteristics and control thereofwhen a preformed spring is used, because they have an efiect similar tovariations in the chofd length of the arcuate operating springs asdiscussed above.

The present invention provides a new improved, lower cost precision snapacting switch combination incorporating a novel normally flat arcuatespring of special design and configuration, which effectively solves theabovediscussed problems in prior arcuate rolling spring switches.

It is an object of the present invention to provide a i new improvedsnap switch which has greatly improved operating characteristics.

It is another object of the present invention to provide an improvedsnap acting switch incorporating a normally fiat arcuate operatingspring of novel design which makes it unnecessary to use preformedarcuate springs, even for close-tolerance precision switch uses. It is arelated object to provide such a switch with such a novel normally fiatoperating spring which eliminates variations in switch operatingcharacteristics and, control thereof heretofore encountered withpreformed arcuate springs due to variations in their chord length. It isstill an other related object of this invention to provide for such snapacting switches a novel operating spring which renders insignificantvariations in compression of the spring during operation, therebygreatly reducing the adverse effect on operating characteristics ofproduction variations, compared to prior switches having preformedarcuate springs.

It is also an object of the present invention to provide a new improvedsnap acting switch including a novel operating spring of flat temperedstrip material, which assumes an arcuate shape upon assembly into theswitch, and has a special configuration such that the flexural stress ofassembled arcuate spring is substantially the same throughout its length(mounting and effects being ignored), and does not significantly exceedthe elastic limit of the spring material. It is a related object of thisinvention to provide such a novel normally flat oper- V ating springwhich will eXert the desired snapover force on the actuated contactmember at the critical snapover position.

it is a further object of the present invention to provide a newimproved snap acting switch incorporating a novel normally fiatoperating spring of special configuration which achieves higher contactpressures andhas better anti-vibration characteristics compared with asimilar switch having a preformed spring of rectangular outline withsubstantially the same operating pressure and travel of the actuatingmember.

It is still another object of the present invention to provide a newimproved snap acting switch and a novel normally flat operating springof special configuration which can be blanked directly from high tensilestrength material, thereby obviating the steps of preforming the springsfrom low tensile strength material and heat treating them to raise theirtensile strength, as has been heretofore done in making preformedarcuate springs. It is performance and uniformity of characteristicswith a substantial savings in production cost that may run as high as ofthe over-all cost of the switch in lower priced,

'more competitive precision switch lines, whereby the present inventionaffords important competitive advantages;

The above-discussed and other objects and advantages of the presentinvention will be apparent from the following description and appendedclaims with reference to the drawings wherein:

Figure 1 is a plan view showing a so-calied closed type snap actingswitch embodying the present invention, 7

with parts broken away to show underlying structure;

Figure 2 is a cross-sectional view along line 2-2 in Figure l; a

F e 3 s a de i plan iew sho ing the actuating,

flat switch operating arcuate spring of this invention,

illustrating the difierence between (a) the total displacement of thespring mounting ends when assembled into a switch and (b) thedisplacement of said ends during overcenter switch operation;

operatively associated with the actuating and actuated members 34 and 36in such a manner that the spring bears upon the abutments or shoulders'42 and 44 during switch operation as hereinafter more fully described.Spring 52 is made of a thin flat piece of resilient metal which is cutto the desired length and has a main portiont54 of novel specialconfiguration according to thepresent invention as hereinafteramplified. The spring 54 has at its ends a pair of mounting cars 56provided with apertures 58 which loosely receive the tongues 38 and 40when the spring is compressed into substantially semicircular shape andassembled in the switch 20 with the ears 56 bearing upon the abutmentsor shoulders 42 and 44 respectively.

Suitable contacts 60 and 62 are arranged opposite the double movablecontact 50 on the opposite sides of the actuated blade 36. The contact60 is mounted on a suitable electrically conductive arm 64 whichis'secured to the sw h base 221W, and in eleet icalcontact with, bushing66 which is provided at its lower end with a terminal screw 68., Thecontact 62 is mounted inlike manner on "a suitable electricallyconductive arm 70 which is mountedon the base 22 by, and in electricalcontact with, a bushing 72 similar-to bushing'66 and pro Figure 6 is aschematic free body diagram illustrating I a normally flat rollingspring under the action of loading imposed on itwhen used in a switchsuch as shown in Figure 1;

Figure 7 is a cross-sectional view taken along line 77 in Figure 6;

Figure 10 is a perspective view showing a modified form of actuated andactuating blade members for use in the switch of Figures 1 and 2; and

Figure 11 is an enlarged plan view showing a modified,

normally flat, constant stress rolling spring according to' thisinvention, incorporating the same principles of design configuration asthe spring inrthe embodiment of Figures 1-8, with modified endmountingtongues, for use with the actuating and actuated blade'members shown inFigure 10. a

Referring particularly to Figures l4,'there is shown at 20 a closed typesnap acting switch embodying the present'inventi'on. Switch 20 comprisesa base 22 of dielectric material, which is provided adjacent one endwith a bore 24 in which there'is seated a metal sleeve 26 having aninternally threaded bore 28'; A screw 30 is threaded into the upper endof bore 28 to secure to the upper side 32 of the switch base 22 anelongated actuating blade 34 which extends within a rectangular openingin an actuated blade 36. Each of the actuating and actuated switchblades 34 and 36 has a tongue or projection 38 and 40, respectively,which extends toward the other. Each of said actuating and actuatedblades 34 and 36also has a pair of shoulders or abutments 42 and 44,respectively, in opposed spaced relation. The actuating and actuatedblades may be formed as a single vstamping from a sheet of flexiblemetal, which is provided a with a hole 46 through which the mountingscrew 30 extends. Alternatively, the actuating and actuated blades 34and 36 may be formed separately if desired, with each having suitablemounting hole like aperture 46 atthe mounting end thereof, in a mannerobvious to those in the art. In the stamping operation, an aperture 48is provided at the free end of the actuated blade member 36 for securinga movable contact 50 thereon.

the assembled switch 20, an arcuate spring 52 is V ated member 6 appliesa three or thrust, action to' id Figure 8 is a'free body diagram of asection of the roll- 3 vided with a like terminal screw, not shown, Aterminal screw. 74 is similarly mounted'in the lower end of bushing 26which is electricallyconnectcd to the actuating blade 34. Thus,electrical current can flow from terminal screw 74 through the actuatedblade 36 and movable contact 50 to either of the fixed contacts 60 or 62and thence to the corresponding terminal screw at the latter contacts.As is well known in the art, if desired to make and break only onecircuit,the contact 60 may be omitted and the arm 64 utilized as a stop.

.Switch 20 is providedwith a cover 76, having sides 7 S which arereceived on shoulders'80 extending around the periphery of switch base22. Cover 26 is provided with a bore'82 in which there is slidablymounted a reciprocable operating, plunger 84, the lower end of whichengages the actuating blade' member34.

As illustrated in dotted line in Figure 2, when the plunger 84 isdepressed, the actuating'lever 34 is moved downward, causing the spring52 to move with a rolling action untilv the actuating lever 34 reaches apoint at which the apertures 58 in spring 52 are substantially inalignment with the free end of actuated member 36, and the ends 56 ofthe Spring are substantially 'at right angles to the actuated member36,. ,At, this point, the actuating member 34 and the spring 52 areovercen'ter critical position, and upon further downward movement oftheactuating m mber 34, the pring end 56 engaging the actumember 36'andfsnaps it upward in Figure 2, thus substantially instantaneouslyopening the contacts 50, 62 and closing the contacts 50, 60. Slots 58have a somewhat greater width than the thickness of the tongues 38 and40 to provide a non-interfering connection, The abovedescribed switchoperation may occur through engagement of the tongues 38 and '40 withthe sides of the cooperating apertures 58 in the ends of spring 52and/or through engagement of the sides of the spring cars 56 against theshoulders 42 and 44 on the actuating and actuated member's,respectively. It will be noted that in the overcenter critical position,the distance between 6a the spring supporting abutments 42 and 44 isless than the distance between thosejabutments in the contact positions,so that the spring is compressed during switch operation. V

The upper surface 32 of switch base 22 is slanted to permit theabove-described displacement of the actuating blade 34. The actuatingblade 3.4 is biased so that when the operating plunger 8.4, is,released, blade 34 returns to its full-line normal position, causing theoperating spring 52 to roll on the abutments 42 andj44 in the oppositedirection and snap the actuated member 36 back to normal positionthereby closing contacts 50, 62- and opening contacts 50, 60.

As discussed in the intrc duction hereof, the arcuate operating springin switches of this kind was heretofore generally preformed to thedesired arc, and in some instances might be made of normally fiatrectangular strip spring material bent into arcuate shape upon assembly;but such prior springs had certain disadvantages as noted above. In thepresent invention, however, the novel improved normally fiat arcuateoperating spring 52 now disclosed in detail provides an improved snapacting switch combination with better precision snap actingcharacteristics which may be more precisely controlled than heretofore.

This improved normally flat operating spring 52 has a novelconfiguration whereby the plan contour, and

hence the cross section, of the spring is varied so thatwhen the springis bent into arcuate form upon assembly in the, switch, (1) the stressin the spring is substantially uniform at all points along its length,and (2) the stress in no part of the spring substantially exceeds. the

elastic limit of the material in either installation or use.

It has been developed that there is an optimum mathe-,

matical relationship between the width at any point along the length ofthe spring 52 as a function of (a) the spring length, ([2) springthickness, and (0) maximum width at the longitudinal center of thespring. And it is possible to mathematically develop and compute theoptimum plan configuration of this new normally fiat spring for a givenmaterial and size.

The principal limiting conditions on the arcuate spring for snap-actingswitches such as here involved, include the following:

The following notations are used in developing the formulas fordetermining the optimum plan configuration of rolling spring 52 so thatthe maximum stress is substantially uniform throughout thelength of. thebent spring when assembled in the switch, the end efiects of ears 56being ignored:

SFlexural stress M-Bending moment tThickness of spring (see Figure 7)b-Width of spring (see Figure 7) 1) ,-width of spring at (i=3 6 Positionparameter (see Figure 6) R-Radius of spring (see Figure 6) PLoad onspring (see Figure 6) a IArea moment of inertia of spring, EModulus ofelasticity ;Figure 6 illustrates the configuration of the subjectrolling spring under the action of loading denoted by P. Thedifferential equation of the elastic curve of this spring is defined bythe equation, I

fl'here is essentially no stress concentration due, to

curvature because the, thickness, 1, is so small incomparison with theradius of curvature, R, of the spring. Accordingly,-the'fiexural stressat any section location defined by 0 canbe computed from the well-knownelementary equation, r

Simultaneous solution of Equations 1 and 2 by elimination of the factoryields the equation,

The factor dy 2 an (dx is the radius of the elastic curve of the springat any point defined by the position parameter 0. For any spring underconsideration in this application, the factors E and t are bothconstant. Thus, for the fiexural stress, S, to remain constant along thespring at any value of 9, it is evident from Equation 3 that the radiusof curvature must remain constant. This means that the elastic curve ofthe spring is a perfect circle. Stated another way, if a spring withconstant B and tis forced to assume a circular configuration, then themaximum fiexural stress will remain constant at all cross sections ofthe spring.

It is now necessary to compute the proper width contour to cause thespring to assume a circular configuration under the action of theloading shown in Figure 1.

Since (from Figure 6) M=PR sin a and Q! dz 1 dy 3,. R nil then Equation1 can be rewritten as sin 0 (6) If I is known at some position, say atthen from Equation 6,

. PR l9.-l'. E Q (7) Equation 6 can now be expressed as I=Il sin 6 (8)Since bt and t is constant, then Equation 8 can be expressed as b=lv|sin 6 (9) 2 Equation 9, then, defines the required width contour.

From the foregoing, the following formula relationships are applicableto design the optimum plan configuration for operating spring '52according to the present invention:

(1 The maximum cessive flexural stresses'so that the elastic limit is noexceeded can be computed from the equation,

where S is the yield point stress of the material. If a a -I. r is knownor assumed. a a

(3) Finally, if desired, the resulting actuating force P can be computedfrom Equation 7.

Following is an example calculation for thedctailed ing to the presentinvention: Data;

,sm,.,=s00,000 p.s.i. R I :0.200 inches ,plan design of a typicalnormally fiat spring 52 accord- E 30,000,000 p.s.i. b =0.250 inches 2SR2 300,000X0.200 (1) E 30,000,000 9 2 b=b a ,xsin a=0.250 sin!)Computational schedule Developed V 0 8 Length, sin@ b Degrees Radians R1 Inches Inches 0 0 0 0 0 10 17453 0349 17304 0434 20 349000 0098 342020856 30 52359 1047 50000 1250 40 698120 1090 04279 1007 50 872650 174570004 1915 00 1. 0471s 2094 86603 2105 70 1 22171 .2443 .93909 .2349 so1.39024 .2792 .98481 .2402 90 1. 47077 3142 1. 00000 2500 3 I} .=b i=L3553 inches Y 3O 1OX1. 3333X10' P-Ellg l.0()0 pound R2 The completeddesign is shown companying drawings.

In designing the novel constant. stress, special corifigurationoperating spring 52 ofthis invention, the elastic'limit or ,proportionalelastic limit is taken for the value of S to compute the varying widthand cross sectional area along the length of the spring by .the

above-disclosed formulas. This eliminates or minimizes setin the spring52 when it is..assembled in the switch, so

' that it would return to normally flat shape if it were in Figure 4 oi-the acthickness'of spring to, avoid ex V springs may have set withinlimits which will provide desired uniformity between springs withrespect to'uniform loading on the actuating and actuated members (34 and36) of the switch. The limit to such set without objectionallyadverse'elfects can be determined'for given switches by routineexperimentation. The spring thickness and/or material (which determinesS can be appropriately changed in the light of the above-discussedformulas to avoid excessive set.

Referring to the schematic drawing in Figure 5, a normally flat springaccording tothe present invention is indicated at 52c. The notationtotal travel designates displacement of the mounting ends of the spring52c from flat position to minimum chord in the overcenter criticalposition" during switch operation, The displacement of said springmounting ends with respect to each other in contact position (e.g., asin Figure 2) to minimum chord in said overcenter critical position isdesignated as "operating travel. It will be noted that the ratio ofoperating travel to total travel? is quite small in the case of thenormally flat, constant stress spring 52 of this invention. However, inprior switches using a-preformed arcuate spring, the ratio of operatingtravel to displacement of the mounting ends of the preformed spring whenassembled into the switch (hereinafter called assembly travel) is large.I Assembly trave is a major determinant of switch. operatingcharacteristics when a preformed arcuate spring is used, so that aconsiderable variation in switch operating characteristics results froma small varation in operating trave e.g., due to tolerance variations instamping of actuating and/or actuated members, 34.and 36, or thepositioningof stops or contacts. .Thus, the new normally flat, specialconfiguration, constant stress spring 52 renders variations in operatingtravel relatively insignificant and. thereby greatly reduces the adverseefli'ects of such variations on precision switch characteristics.

Similarly, in prior switches incorporating preformed arcuate springs,small production variations in the chord between the mounting portionsof the spring (corresponding to the slotted portions 56 of spring 52),introduce versely afiects operating characteristics. As analternasubstantial diiferences in switch characteristicsfsuch as contactpressure, operating force, and resistance to .contact chatter undervibration. Likewise, fora given spring chord, substantial differences inoperating characteristics are introduced by small variations in thespacing between the spring supporting abutments 42 and 44 due toproduction tolerances, stamping of the actuating and actuated blades,and in the positioning-of blade stops, In the newimproved switch of thepresent invention embodying the above-described novel constant stress,normally flat spring 52, these shortcomings are substantially eliminateddue to the small ratio of the size of such variations to the totaltravel of the spring ends.

' With prior preformed arcuate springs, as the ratio and snap actingthrust exerted on the actuated blade are both increased. Therefore, anincrease in' this ratio has sometimes been resorted to in orderto'increase the contact pressure, snap acting spring thrust, ,andvibration resistance in prior arcuate spring switches of a given size.However, this expedient increases the stress at the center of the.spring during switch operation and frequently results in a bend in thespring which adtive, therefore, designers sometimes resorted to athicker spring in which this chord ratio was smaller. However,

' in practice it is found that use of a heavier spring precise controlrequired for close tolerance precision.

creates problems because its'force is not subject to the switches,excepting when the spring operates between very close limits. Anotheradvantage of the novelstress-eontrol, normally flat, operating spring 52of this invention is that it is possible to provide an increased amountof fiexure, with resulting lower operating force and higher contactpressure, without over-stressing the spring and thereby avoid adverselyafiecting operating characteristics.

Higher forces are achieved with material of the same thickness forconstant stress springs of this invention as compared to the preformedorchorded. This is because all of the spring is worked at high stresswhile the preformed or chorded spring has high stress only in localizedareas thus much of the spring is not fully utilized. From an energypoint of view, more energy is stored in the constant stress spring ofsimilar operating force because it has been compressed through a largerdistance. Likewise more energy is available to propel switch parts sincethe force does not fall off as rapidly with expansion as for thepreformed or chorded spring.

This invention thus achieves a-wider range of control of switchcharacteristics than was possible with prior preformed arcuate springs.

It has been found that the contour of the novel controlled stress,arcuate spring 52 of this invention (i.e., the width at given pointsalong the spring for a given material, spring length, and thickness) canbe varied slightly from the optimum dimensions determined by theabove-disclosed formulas, while still retaining the essential featuresand advantages of the above-disclosed invention. The degree that thewidth at each point along the length of the spring can be varied fromthe optimum width computed according to the above formulas, and moreespecially Equation Number 9, can be determined by routineexperimentation and is governed to large extent by the factors discussedabove in connection with slight permissible set.

In commercial practice, the springs, 52 are stamped to their specialoutline with dies of corresponding contour. In order to use lower costdies, the spring configuration plotted according to the above formulas,especially Equation 9, is substantially reproduced as closely aspossible by a composite curve made up of a series of circular arcssubstantially coinciding with the plan contour of said spring accordingto Equation 9; The stamping dies thus can be cut on a radius, accordingto practices known in the die making art.

It will be apparent that the ears 56 provided for mounting the spring inthe switch necessitate some departure from the optimum springconfiguration according to the above-discussed formulas. To a certainextent, end effects due to provision of these ears may be ignored. Thesize of ear dimensions designated at and y in Figure 9 is important,however, especially in small size springs, e.g., a ear on a ,3 maximumwidth spring has a much greater effect on switch characteristicscausing. more of a departure from the theoretical optimum than a ;ij""ear' on a spring of .375 inch maximum width. The maximum size of theear can be determined by empirical tests. In most switches the size ofthe car can be designed as the minimum to satisfy mechanicalrequirements based on the dimensions of tongue receiving slots 58(Figure 4).

In Figures and 11 there is shown another embodiment of this inventionwhich illustrates that the desired variation of spring width, and hencespring cross section, along the spring length, may be obtained byspecific means difiering from that employed in the abovedescribedpreferred embodiment of Figures 1-9, but utilizing the same principle.For clarity, parts of this embodiment corresponding to like parts in theembodiment of Figures 1-9 are identified with like numerals plus thesubscript a.

Referring especially to Figure 11, there is shown a spring ofrectangular outline 52a with a central portion removed leaving anaperture with curved sides having a configuration plotted according toFormula No. 9 above,

As will be apparent from Figure 11, for each value-oil R9 (proceedingfrom the spring ends towards its center) a distance equal to accordingto Formula 9 is plotted from the sides of the spring towards the center,to develop the side curves of the central cut-out portion. The endmounting portions 56a are provided similarly to mounting ears 56 in theFigures 1-9 embodiment, and have end mounting notches 58a. The distancex from the center line of notches 58a and the mounting ears 56a ofspring 52a is determined by routine experimentation according to theprinciples discussed above in connection with thesize of the dimensionsat for ears 56 in the embodiment of Figures 1-9.

It will also be apparent from Figures 10 and 11 that: the new andimproved snap acting switch and spring; according to this invention neednot use a tongue and groove connection such as in the switch embodimentof Figures 1-9. Thus, for example, referring to Figures 10 and 11, thespring 52a is provided at its mounting ends 56a with a plurality ofnotches 58a extending in- Heretofore, preformed arcuate springs forswitches of this type have been formed to the desired arcuate shape froma low tensile strength material of say 100,000 p.s.i., and thenheat-treated to raise the tensile of the spring to approximately 180,000p.s.i. in a typical design. This heat treatment varies the spring chordand thus the forces the spring will exert on the actuated and actuatingmembers in an assembled switch, thereby causing variations in operatingforces and characteristics, from spring to spring in a givenheat-treating batch, and from batch to batch. This method of springproduction has been used heretofore because it has not been practical topreform arcuate spring directly from high tensile strength material tothe desired chord. However, the novel configuration, constant stress,normally fiat arcuate spring of this ,invention is blanked directly'fromhigh tensile strength material such as 180,000 p.s.i. beryllium copperor 300,-

000 p.s.i. Elgiloy. Thus, besides providing more uniform switchcharacteristics, which is the primary aim, this invention results insubstantial production savings due to these process simplificationsalone.

The novel special configuration, stress-controlled, arcuate spring ofthis invention provides substantial savings in spring production costswhich cut the over-all cost of an entire switch unit as much as 10%,especially in lower priced, more competitive switches.

The springs 52 or 52a of this invention are bent into arcuate form uponassembly into the switch and special tools may be provided for doingthis.

It will be apparent from the foregoing that the present inventionprovides a new improved rolling spring snap switch incorporating a novelimproved, constant stress, normally fiat, arcuate spring of special planconfiguration and predetermined cross sectional variation along itslength; which achieves new improved results and advantages in precisionswitch operating characteristics and control thereof over prior arcuaterolling spring switches; and that the same solve important shortcomingsof such prior switches, and achieve other objects and advantages asdiscussed in the introduction of this specification, and otherwisedeveloped herein.

The invention may be embodied in other specific forms 11 withoutdeparting from'the spirit or essential characteristics thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative andnot re strictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.a v a What is claimed and desired to be secured by United States LettersPatent is: V r 1 1 A snap acting switch comprising: a switch body; anactuating member on said body and having tongue means on'one end; anactuated memberton'said body,

portion being larger than and loosely receiving the tongue; means ofsaid actuating and actuated member whereby movement of said actuatingmemberfcauses said spring to move with a rolling action and whereby uponactuation ofsaid actuating member through critical position insubstantial alignment with said actuatedmember, the spring snaps saidactuating member and the movable contact thereon in a direction oppositeto the movement of the actuating member to efiect switch operation; saidspring, being unformed prior to assembly in the switch and having a plancontour such that the width of spring material progressively decreasesalong its length from a maximum at the center thereof so that'no part ofthe arcuate spring substantially exceeds the elastic limit of the springmaterial and said spring has asubstantially constant bending stressthroughout its length, end effects where 't is'the thickness of thespring, S is the yield point stress, of the spring material, R isttheradius describing the arcuate form of said operating spring, and E isthe modulus of elasticity of the spring material. 7

3. A snap acting switch as defined in claim 1, wherein I the plancontour is such that the width at each point along the, length of thespring excepting at said mounting 12 portions is substantially the valueof b calculated according to the equation,

and where 0 is the angle defining the position parameter of saidoperating spring. 1 i

4. A snap acting switch as defined in 'claim 3, wherein the centralportion of said, spring has a double convex plan outline with endmounting ears.

5. A snap acting switch as defined in claim 3 wherein the centralportion of said spring is cut out providing an aperture of doubleconcave plan outline.

'6. A snap action device for a switch comprising an actuating member, anactuated member, a flexible operating spring having a semi-circularportion operably disposed between said members and mounting endportions,

abutment means operativelyinterconnecting, each mounting end portionwith adjacent ends ofsaid actuating and actuated members wherebymovement of actuating member causes said spring to move with a rollingaction, means for moving said actuating member through a criticalposition relative to saidactuated member whereby said spring causes saidactuated member to move with a snap action, the semi-circular portion ofsaid/spring having a plan contour of varying width which decreases froma maximum at its center to a minimum at the end portions to produce asubstantially constant bending stress in the semi-circular portion ateach point along its length, and the width of the semi-circular portionvaries according to the equation 1 r b.=b sin 0, Where b T is themaximum width of the spring, where b is the width at a positionparameter along the semi-circular portion and 0 is the angle measuredfrom one of the end portions to such position parameter. v 7

7. A snap action device as defined in claim 6 wherein the semi-circularportion of said spring has aldouble convexplan outline.

8. A snap action device as defined in claim 6 wherein the semi-circularportion of said spring "is cut out providing an aperture-of doubleconcave plan outline;

References Cited in the file of this patent UNITED STATES PATENTS GreatBritain ,Mar. 3. 1947

